LPG is stored in tanks located outside the buildings using this form of fuel. To ensure that an adequate supply of LPG is available to the building, each tank must be periodically refilled by a fuel delivery to the tank location. Fuel requests are placed with a central distributor who makes the deliveries utilizing tank trucks.
Automatic fuel reordering systems alleviate the consumer's inconvenience of periodically checking the fuel level. In addition, by utilizing these systems, distributors operate more efficiently and competitively. Such fuel reordering systems detect when the fuel reaches a particular level and, upon detection, transmit an informational message, via the building's telephone lines, to the distributor. By knowing when and in what quantity a consumer requires fuel, the distributor may more economically schedule fuel deliveries to his or her clients. In general, deliveries will be made only when a consumer requires a substantially full tank-load. An example of a fuel reordering system is disclosed in U.S. Pat. No. 4,845,486.
To date, fuel reordering systems have been substantially limited to fuel oil systems. In one prior art example, the fuel level is directly sensed by a capacitive sensor. This type of sensor employs concentric cylinders that are inserted into the fuel tank. The fuel oil forms a portion of the dielectric between the concentric cylinders, and air within the tank forms the remainder. As the level of fuel oil decreases, the fuel oil comprises a lesser portion of the dielectric, thereby changing the resulting capacitance of the cylindrical capacitor. An electronic circuit compares the resulting capacitance with a reference capacitance corresponding to the particular level of fuel to be detected. (The particular fuel level to be detected is hereinafter referred to as a "watchpoint.") When the capacitances are relatively equal, the circuit conveys an informational message to the fuel reordering system, so that a fuel request can be made to the distributor. In fuel oil systems, directly sensing the fuel oil level by capacitive sensing, or other equally active and invasive methods, represents relatively little danger as fuel oil is non-explosive.
The prior art has employed indirect sensing of the fuel oil level under limited conditions. For example, in fuel oil systems utilizing a mechanical, sliding float gauge, the position of the mechanically sliding indicator element is sensed, rather than sensing the fuel level itself. A magnet is affixed to the indicator element, and a magnetic reed switch is attached to the fuel gauge in a position corresponding to the desired watchpoint. The reed switch detects when the magnet comes within a certain proximity, and upon detection, conveys an indicative signal to the fuel reordering system. This sensing method is limited to only certain types of mechanical fuel oil gauges and is substantially limited to fuel oil systems. Only certain gauge types allow the reed switch to be attached and also allow access to the gauge's indicator element so that the magnet can be affixed. This method of sensing, with its concomitant gauge modifications, is acceptable for fuel oil systems, as there is a negligible risk of explosion in such systems. See U.S. Pat. No. 4,845,486 for more discussion on prior art reordering systems and their sensing methods.
LPG systems have more demanding requirements than fuel oil systems. LPG tanks and LPG gauges, which are attached to the tanks, are located outside buildings. Consequently, LPG gauges must withstand extreme environmental conditions. In addition, because LPG is stored under pressure and is highly explosive, a LPG system must ensure that the gas does not leak or inadvertently flow into the building. A stuck valve or gauge can result in catastrophe. The LPG sensor must be able to meet these same stringent demands and not add any risk of leakage or explosion.
Capacitive sensing is unacceptably risky for LPG installations. First, the LPG could experience a capacitive discharge and explode. Second, because the sensors require openings in the LPG tank for the capacitor apparatus, leaks might result and cause an explosion.
Due to the constraints involved, currently known sensors are unacceptable. Most of the currently known sensors do not adequately address the safety concerns of a highly explosive fuel and extreme environmental conditions. Some require access to the gauge's internals, thereby increasing the cost and difficulty of installation. Others require the existing LPG gauge to be replaced, thereby significantly increasing the cost. For example, a sensor device for use with LPG systems is manufactured by Squibb-Taylor, 10807 Harry Hines Blvd., Dallas, Tex. 75220, under the name of the ATLAS Monitoring/Reporting System. The ATLAS system requires replacing the entire LPG gauge indicator element with the sensing apparatus. The sensing apparatus is therefore more complicated since it must include indicator element functionality. Further, the gauge's original indicator element is wasted. Lastly, most known sensors are incompatible with the predominant LPG gauge in the market, the Rochester 6200, manufactured by Rochester Gauges Inc. of Texas, located at P.O. BOX 29242, 11616 Harry Hines Blvd., Dallas, Tex. 75229.
The Rochester gauge shown in FIG. 6 is comprised of two self-contained, hermetic units: the base unit 157 and the indicator unit 150. The base unit 157 is attached to the LPG tank and includes a float mechanism 158 to be inserted into the tank. The float mechanism 158 is responsive to the level of liquified fuel within the tank. Attached to the float mechanism is a rod 600 and gear mechanism 602 that rotates responsively to the position of the float mechanism. A base magnet 604 is transversely attached to the distal end of the rod 600. Thus, the angular orientation of the base magnet 604 corresponds to the amount of fuel within the tank.
The indicator unit 150 visually indicates the fuel level. This unit is aligned with and secured to the base unit by screws 340. The indicator unit 150 includes an indicator magnet 606 that is connected to an indicator element 152. Much like a compass, the indicator magnet 606 couples with the base magnet 604. The indicator element 152, being attached to the indicator magnet 606, is thereby responsive to the fuel level.
As previously mentioned, the LPG system is potentially subject to extreme environmental conditions. The magnetic coupling between the base and indicator magnets (604, 606) allows each unit to be sealed. Sealing each unit minimizes the risk of water or moisture freezing within a unit and thereby freezing it. Separate hermetic units also minimize the likelihood of LPG leaks. For example, in the Rochester gauge, only the base unit 157 presents any risk of fuel leakage. (Hereinafter the Rochester gauge is referred to as the "LPG gauge.")
Accordingly, it is an object of the present invention to provide a sensing unit capable of detecting when a LPG gauge indicates that a particular fuel level has been reached.
It is another object of the present invention to be compatible with existing LPG gauges and fuel reordering systems.
It is another object of the present invention to provide a LPG level sensing unit that is easy to install and safe.
It is a further object of the present invention to achieve a low component count by utilizing the functionality of the LPG gauge.